1 . Technical Field
The present invention relates to sheet processing apparatus included in an image forming system for xerographic printing; more specifically, the present invention relates to sheet transportation apparatus comprising a sheet transport guide provided between image printing and image fixing units of the image forming system, and further relates to a ensuring normal and smooth operation of the image fixing unit in its sheet transport function, and to a a mechanism associated with the sheet transportation apparatus for facilitating the remedying of sheet jams ocurring therein.
2. Description of the Background
(1) An image forming system incorporated into such equipment as a copying machine or a facsimile terminal includes an image printing unit for transferring a toner-developed image onto a printing sheet, and an image fixing unit for fixing the toner image onto the sheet. Between the image printing unit and the image fixing unit, there is a sheet transport means for transporting the sheet bearing the unfixed toner image.
While the sheet bearing the unfixed toner image is transported to the image fixing unit, it is necessary to prevent the unfixed toner image from being damaged. Therein, Japanese Patent Publication No. 59581/1989 discloses that the surface resistivity of a guide member disposed on the sheet-discharging side of a transport belt be set between 1.times.10.sup.6 .OMEGA. and 1.times.10.sup.9 .OMEGA.. In the above disclosure, since the guide member for guiding the transport of the sheet has a predetermined electric resistivity, electric charge carried by the sheet is not abruptly drained, whereby the unfixed toner image is not likely to be disturbed.
Accordingly, because the guide member guiding the sheet to be nipped by fixing rollers has a predetermined resistivity, is likely to carry a charge of the same polarity as that of the sheet. In this case, the leading edge of the sheet reaching the guide member could be repelled upward away from the guide member by electrostatic repulsion, whereby the leading edge of the sheet, being not properly guided to the nipping position of the fixing rollers, is caused to collide with the surface of the heating roller. This collision with the heat roller can disturb the unfixed toner image degrading image quality.
(2) In the image forming system, a sheet transportation apparatus is provided between the image printing unit, including a photoconductive drum, and an image fixing unit. A transport guide surface of the sheet transportation apparatus is bent so that the sheet conformingly bends as it passes along it and the sheet is smoothly guided to the nipping position of the fixing roller pair. This curve in the sheet absorbs the impact mentioned earlier, when the sheet collides with the heating roller. Damage to the image is thereby prevented.
Conversely, when the trailing edge of the curved sheet is released from a sheet-feeding resist roller pair, the curved sheet, flexes straight due to its stiffness, thereby pushing the paper backward through developing rollers of the image printing unit, which can damage the image in the image transferring region of the sheet. Japanese Patent Application Laid-Open No. 293272/1987, to prevent this reverse movement proposes to reduce the curve by setting the transport speed of the fixing roller pair higher than that of the resist roller pair.
When the difference of the transport speeds of the fixing roller pair compared to that of the feeding roller pair is too small, the sheet is not completely straightened and therefore the damage to the image is not sufficiently prevented. On the other hand, if the difference between both speeds is too large, the degree of magnification of the image differs before and after the leading edge of the sheet is nipped by the fixing roller pair and before and after the trailing edge of the sheet leaves the resist roller pair. Consequently, the image is distorted and can not be precisely reproduced.
(3) The image fixing unit in the image forming system includes a heating roller having a heater inside and a pressure roller pressed against the heating roller. As the sheet is transported between the rollers, the toner image on the sheet is fixed. Both ends of the heating roller of the fixing apparatus are rotatably supported by bearings. A power transmitting element such as a sprocket or a gear is fixed to the exterior side of one bearing. The power transmitting element is normally disposed on the rear side of the system for maintenance purposes or the like. An insulating member formed of a synthetic resin has been recently used for the power transmitting element in order to reduce the cost and weight.
A cooling fan is provided in the image forming system. Cooling air from the fan flows in a stream from the front side to the rear side of the system to prevent the heating roller from overheating various unit components. Since the stream of the cooling air runs along the heating roller, the surface temperature of the heating roller is high on the front side and low on the rear side.
Since the power transmitting element is disposed on the rear end of the heating roller (downstream of the cooling air) and in the rear of the system, and because the power transmitting element formed of a synthetic resin serves as an insulating member, heat from the heater is not likely to escape outside the system on the rear side. Consequently, heat emission becomes uneven, i.e., the temperature at the rear end of the heating roller is higher than that at the front end.
(4) In the image fixing unit of the image forming system, a thermistor for detecting the surface temperature of the heating roller is mounted in a housing covering the heating roller. Temperature data detected by the thermistor is input to a temperature controller for regulating the temperature output of a heater inside the heating roller. A temperature fuse is also provided in order to prevent the heating roller from becoming too hot due to a malfunction of the temperature controller. The temperature fuse is disposed close to the heating roller and melts when the heating roller is becomes abnormally hot, thereby interrupting supply of power to the heater in the heating roller.
Recently, it has been proposed that the housing covering the heating roller be formed of a heat-resistant resin (Japanese Patent Application Laid-Open Nos. 101780/1991, and 297578/1990). When the housing is formed of the heat-resistant resin, heat retaining properties and fixing performance during the image fixing operation are improved. Furthermore, the temperature of the system is prevented from becoming abnormally hot so that damage to the system is more likely to be minimal.
However, when the image fixing temperature runs hot out of control due to system malfunction or to a wiring error for example, the temperature could rise above the heat-resistant temperature of the resin which forms the housing. The housing will then thermally deform to an extent that mounting positions of the thermistor and temperature fuse could shift within the housing. If the thermistor separates from the heating roller due to thermal deformation of the housing, for example, the temperature it detects would be lower than the actual surface temperature of the heating roller, such that power would be further supplied to the heater, increasing the overall damage. Furthermore, if the temperature fuse also separates from the heating roller, the temperature fuse will not melt at the critical temperature. Consequently, the fixing apparatus would overheat, bringing about further damage to other components in the unit as a whole.
(5) A discharge roller unit for discharging sheets to the discharge tray is provided on the sheet-discharging side of the image fixing unit within the image forming system. The discharge roller unit includes a pair of upper and lower rollers. The sheet is nipped between both rollers and transported to the discharge tray which is disposed outside the system.
When a sheet gets jammed between the pair of rollers of the fixing apparatus or the discharge rollers, the jammed sheet can be readily taken out by releasing pressure between the rollers or by releasing the driving force on each roller. Conventional systems employ devices wherein the upper and lower frames are openable, the heating roller and the upper roller of the discharge rollers being mounted in the upper frame, and the pressure roller and the lower roller of the discharge rollers being mounted in the lower frame. In a sheet jam situation, the upper frame is opened so that the heating roller and the upper discharge roller are parted from the respective rollers beneath. Consequently, the sheet jammed in the discharge portion can be easily taken out.
As described above, upper and lower rollers in this conventional structure are likely to become offset from their original alignment, causing the sheet discharging function to be degraded. For this reason, other conventional system approach jammed sheet removal without displacing the rollers. A magnetic clutch or a one-way clutch is provided between the discharge roller pair and a roller-driving power train, for releasing the driving force on the roller pair. The magnetic clutch is turned off in a sheet jam situation in order to release the driving force on the discharge roller pair, at which point the jammed sheet can be readily taken out. Alternatively, the one-way clutch readily allows jammed sheets to be removed in a single direction.
However, the magnetic clutch and the one-way clutch are expensive, increasing the costs of the image forming system. In addition, since the jammed sheet is removable in only one direction when the one-way clutch is provided, it is difficult to remedy the jammed sheet in some apparatus.